my iMac seems to have seized up.

using my iMac I needed to open a desktop folder to access a file stored within. The folder has many files, I access frequently to refer or update. on this occasion the folder was completely empty.

The document I needed is an Excel spreadsheet. I did manage to access via the Excel programme. I received an on screen message the I might force quit to solve so I closed Excel in that manner.

I then tried to restart which resulted in approximately 1 hour of bright screen then sudden appearance of sign on screen. Sign in normally my folder seems to be back to normal..

I now have repeated pop ups requesting key chain password. I am am stuck there. it's not accepting keuchain password at the moment. is it possible to reset it?

Thanks