goodmans gnav12, software fault?

I am a multi-drop driver and my goodmans gnav12 decided it didn't want to work today, luckily it was very quiet today (delivery wise). Its out of guarantee and the software not the latest but has worked fine till this morn. When u turn it on, press the navigation bit on screen a message appears.......CurrentMapDataChecking error :Tab file is corrupted! What does that mean?, I tried the reset button which didn't help. The map version is UK_Ireland Version: 2006.Q1.4156 I use it daily for work and is a godsend. Any help is appreciated, is there a master reset back to initial factory settings or sommat?. Thanks guys