What if I forgot to bring my poster to the conference?

Print it locally.

Hopefully, your conference is in a city of some non-negligible size and not in the middle of nowhere. Find a copy shop in the city and print your poster there.

Large conferences may even have their own on-site printing, although a last-minute print job may be expensive or even unavailable.

Some people do this in any case, so that they don't have to travel with a poster.

(Your university/employer may or may not be willing to pay for a 2nd printing of the poster)

There are two levels of issues that you may have to cope with.

First, let's look at what to do at the conference:

You already suggested some options:

• just talk about my work

This might serve as a last resort fallback, but it probably does not leave too good an impression.

Maybe more importantly, talking about your work might be more difficult than expected, because without a poster, you won't be recognized as a presenter. In my experience, poster sessions usually work in a way that the audience slowly wanders around, looking at poster after poster. If someone from the audience has any questions or wants some explanations, they will look for someone standing around next to the poster they are currently reading. Hence, if there is no poster, no-one might stop to talk to you in the first place, because you will be assumed to be just another member of the audience looking at the posters around or vice-versa of you.

• present it on my laptop/tablet screen

That might be an option. Maybe you should make up for not providing a large poster by being ready to also run a small demo of your work, if possible, and if allowed by the conference.

If you happen to carry around a portable projector, you could even truly replace your poster.

• print it on an A4 sheet,

In this case, I suggest printing tiles of it on several A4 sheets, so you can reassemble the poster.

As suggested in gerrit's answer, you can of course also try and find a way to print the poster again in its original size, which I too find (visually) preferrable.

or get a larger sheet (e.g., from a flip-chart) and draw the outline and handwrite the key parts of my poster

That's quite a valid replacement.

• withdraw from the conference (no poster, no presentation), but some people may be wondering why I'm not there.

There are always some planned attendees who are not there (visa issues, ...). People who have already met you during the conference might indeed wonder, but a simple explanation like "unforeseen change in plans" should be sufficient for them, if anyone even asks.

However, as there are various workarounds, as described above, the possible repercussions from this might be too severe as to not try and use the workarounds.

Second, there is the question about your home institution:

First of all, the poster is (probably) university property, paid for by the university. Some universities might not care about getting back the posters, others do. In the latter case, things are not much different than losing your office laptop, or any other piece of equipment. (In fact, along with the poster, you probably lost a university-owned poster tube.) Unrealistic as it may be, these pieces of equipment are often not insured for business trips, or at least for trips abroad.

As such, be ready to provide a replacement. I am not saying your university, or your department, or direct superior, could not be lenient if this happens once. But be prepared that you may not get reimbursed for printing another copy of the poster at the conference location.

Then, there might be the more severe issue that occurs if you went to the conference with only the poster to present and nothing else. Attending the conference is probably worthwhile for you just as well, but chances are the business trip to the conference was formally permitted because you were "required" to attend in order to present your poster. This is why I would urgently advise against the option to withdraw your submission, if there is any other way, as otherwise, you may be found to have violated the basis upon which your trip is funded and thus end up paying the whole trip and conference fee yourself.

I have actually done this before -- finally managed to get the blasted thing printed out by about three in the morning, shambled home, left the poster tube on top of my bag, and still managed to leave it behind when I left the next^W later that morning. There's a longer version of this story which involves much running around a building with flapping curtains of A0 paper, and mucho, mucho swearing.

Anyway, I managed to find and download a 'split up your postscript into A4' program, over the piece of wet string that claimed to be the rather remote conference's internet connection (no copy shops...), and with much help and sticky tape from the conference reception, assemble this into the original(ish) poster.

Upside: I had a poster that stood out! And a funny story to tell, which together meant that I probably got a rather larger and more sympathetic audience at the poster session than the (in retrospect rather dull) poster actually warranted.

If there's a moral to this, it this: conferences are social occasions, with human people at them. And if you can make a connection any way – whether it's a weird-looking poster, or a ‘here's me improvising with my tablet’ performance – you'll potentially make a bigger impact than if everything goes smoothly (well, that's what I told myself).