You have two options to try to fix this problem.
1. Reset the password in the Joomla back-end by editing the user's settings in Joomla's User Manager, or
2. Reset the password in your database, directly.
I had to use the latter method just the other day because I wasn't able to click the Save & Close icon in the User Manager after entering the new password.
Resetting the password in the database is the same process described in an older article titled "Locked Out?: Resetting the Joomla Admin Password" except that instead of using the MD5 hash for what used to be the default Joomla password, "admin", you would create an MD5 hash for your desired password and use that instead.
To create the MD5 hash, search on the web for "md5 hash" and choose one of the free online MD5 Hash generators. I use Miracle Salad's MD5 Hash Generator.
Once you've got your MD5 hash created, select and copy it and save it in a text file.
Then open up the Users table in your database manager tool, find and select the record for the user having the problem, and paste the hash into the Password field. Click the Go button at the bottom to save it, and go test it by trying to login to Joomla.
Hopefully it worked for you. If not, I'd try it a second time to make sure you copied the hash correctly. If it still doesn't work, then backup your site files and upload a clean version of the Joomla core files, overwriting the current files in the process.