Why does intellisense and code suggestion stop working when Visual Studio is open?

I should note that I haven't had the issue since upgrading my RAM. I can't confirm if it's related but the problem was prevalent when I had 2-4GB RAM. No problems since going to 8 and 16GB.

If only one file/window appears to be affected, close and reopen that file. If that doesn't work, try below.

In Visual Studio:

1. Click Tools->Options->Text Editor->All Languages->General
2. Uncheck "Auto list members"
3. Uncheck "Parameter information"
4. Check "Auto list members" (yes, the one you just unchecked)
5. Check "Parameter information" (again, the one you just unchecked)
6. Click OK

If this doesn't work, here's a few more steps to try:

1. Close all VS documents and reopen
2. If still not working, close/reopen solution
3. If still not working, restart VS.

For C++ projects:
MSDN has a few things to try: MSDN suggestions

The corrupt .ncb file seems most likely.

From MSDN:

1. Close the solution.
2. Delete the .ncb file.
3. Reopen the solution. (This creates a new .ncb file.)

Notes:

• Tested in VS 2013/2015

Logging possible causes:

• Copy/pasting controls in a source page. I found that my designer.vb file didn't update from this, either.
• Copy/pasting code from another page that caused an error because the code copied referred to a control that wasn't on the page I was pasting to.
• C++ project has corrupt .ncb file

(Please add to comments if you notice behavior that causes this.)

What works for me is removing the dynamically-built .suo file (Solution User Options), in the .vs (hidden) directory located at he same path as the solution file.

I have this problem sometimes coming back, and it's on different project's/solutions, but never VS wide. New Projects always work fine.

Running VS2015 Professional Update -2-

I am having the same issue; Intellisense randomly will stop showing in some files, but not others. I just had it happen to me again. Hitting Ctrl + Space won't show anything in Form1, switching to Form2 or any other class will pop up the list as expected. Restarting Visual Studio usually does the trick, though it's highly annoying and ridiculous for such a basic feature to be broken...