tom tom 5 +hp rz1710 problems

not sure if anyone can help but i have an rz1710 and tomtom5 but occasionaly tomtom loses its memory it wont list any favourites, this morning it said no cities stored? then it came up with error planning route. luckily i new where i was going (work) so i didnt get lost however when i got to work it the planned my route for me. does anyone have any ideas ??.
forgot to mention i may not be able to ask tomtom for their advice if you know what i mean