Below alerts are very critical alerts but its depends on the hardware, when external hard disk (SAN) is unplugged or handled in properly and sql server could not located the drive you may get error like this.
Alert description: Error: 823, Severity: 24, State: 2 I/O error 38(Reached the end of the file.) detected during read at offset 0x00000044c10000 in file ‘d:\Program Files\Microsoft SQL Server\MSSQL\data\tempdb.mdf’.
Alert description: The operating system returned error 21(The device is not ready.) to SQL Server during a read at offset 0x00000000196000 in file ‘E:\MSSQL\TempDB_Data1.MDF’. Additional messages in the SQL Server error log and system event log may provide more detail. This is a severe system-level error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.
sometimes: resource database data and log files should be on the same location as master database- require for sql server 2005 which taken care in sql server 2008
Check the sql server and its database are everything is good
Check intern drives on the back are all required drive are visible
Check eventvwr or errorlog.txt and see if you could find message that the required drive is missing.
Contact your it support/ server administrator and make sure all the required drives are visible to you.
Also if all the drives are good, could be hard drive is corrupt. We may need to try to fix it. and if its hard drive issue- the only other option left for you is to restore your last good backup.