this is frustrating to see and if it happens quite often. There can be anyone of reason listed below:-
- You have migrated from a database that you were using for other domain or subdomain.
- Memory/ cache storage / Session is filled up and not able to take it anymore.
- an Extension was recently installed.
What can be done (First thing, Make sure you have taken backup for safety purpose, this can lead to unmanageble situations, Refer 5 ways to take Magento website backup)
- Solution One:
- Try to Login via a private window of your browser. There are chances it will work. Once you get inside the admin dashboard, Go to System –> Cache management -> select all cache –> Flush Magento Cache.
- Login into your server via ssh using putty or SecureCRT (assuming a linux server)