Databases not accessible

Just got a call that three databases on an instance where many reside were not accessible. Users were getting errors about not enough disk, memory, or the files were inaccessible. I logged into the server and found that the databases were not offline but could not be accessed. I looked into the errorlog and found the following error:

‘The transaction log for database ‘database_name’ is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.databases’

Well I looked around and I did not find anything wrong. Disk were logs are kept had just under 100 Gigs free and the ldf files were not in a read-only state. Based on that I offlined the databases and then attempted to bring them back online. SQL found the MDF and the LDF files, went through recovery, found no issues and brought the databases back online.

Still not sure exactly what happened. It is strange also since all the databases use the simple recovery model. Based on that I am not sure how the log was full but there you go.