On adding components in Eagle schematic same are not reflected on Eagle board else link error pops-up?
To maintain synchronization, once the board is generated from the schematic, you have to have both open when modifications are being done (particularly the schematic).  EAGLE does this automatically under normal circumstances.  Unlike some other programs, the board netlists, and components are created/modified/deleted as you work on the schematic.

Since both files (as of release 5.0, IIRC) are XML (text) based, you can go in and modify each file by hand to get them to re-synch.  Depending on how much has changed this can take a bit of work.  If you haven't done any major layout work on the board, rename the .pcb file (just in case) and create a new board file from the schematic.

EAGLE also maintains a backup set of files which you can use to recover the work (as long as you haven't gone too many iterations).  You can also use the ERC function to get an idea of what it's complaining about.