I installed a plugin that compresses images, then I deleted it because it didn't work. Then, I go to log into my admin panel the next day and it says
invalid username or password
you can login to phpmyadmin, search for "admin_user" field & click on that and choose MD5 and give new password & save it.
step 1 : Login to Phpmyadmin & select proper database
step 2: : search for admin_user
step 3 : click on admin_user
step 4 : select MD5 from dropdown and type new password
step 5 : click on "Go" button
you can find detailed explaination here .