Eagle CAD: method of re-linking schematic and board?

From schematic run ERC. Consistency errors will be at the top. Fix the errors by removing parts, nets, etc. that causes them then try to switch to layout and back. Repeat until the error message disappears.

Alternatively, some versions of Eagle generate .b#1, .b#2 and .s#1, .s#2 files which are backups. If you rename the extension to .brd and .sch you can go back a few steps and hopefully recover what you lost.