Which version of INFORMIX?.. SE?.. IDS?
I had this problem with SE because the tablename in SYSTABLES did not match the file (tablename.dat and tablename.idx) stored in the dbname.dbs directory, so I had to rename the .dat and .idx filenames to matchup with the tablename stored in SYSTABLES, then DROP TABLE tablename so that it could find the renamed .dat and .idx files. The other posibility is that the tablename doesn't exist in SYSTABLES although the .dat and .idx files are present. You can also login as user "informix" and update the row in SYSTABLES for the table in question but that is risky! Remember there also columns associated with the table in SYSCOLUMNS, indexes in SYSINDEXES and other dependencies.