It is likely that this means blockchain database (LMDB) corruption.
The easiest way to fix it is to rename your corrupt
lmdb folder to something else, for example:
C:\ProgramData\bitmonero\lmdb-old and run
It should start without errors and it will again create a fresh copy of
C:\ProgramData\bitmonero\lmdb and sync from scratch. It will take some time to catch up with the network again, though.
Also, if you don't mind, please make a .zip archive of the
C:\ProgramData\bitmonero\lmdb-old and upload somewhere for Monero dev. hyc, who is the creator of LMDB database, to examine (don't worry, your wallet data is not stored there, it's safe to share). This will help to understand why it had been corrupted and possibly prevent such occurances in the future.
At the end, when all is working, you can safely delete the
Note: the example above is for Windows systems and default settings. On other systems, you'd need to locate your
\lmdb folder and do practically the same, just at different folder location.