Hi,how long have had it for because you might still have a warranty. if u you don't i suggest buy i knew one since usually fixing cost as much as buying a new one.
You're not putting valid DVD in the unit
Check that your discs are clean, don't have grease smudges, scratch's etc and are a playable media it accepts to be played.
Dusty lens on the reader can be a cause, so use a DVD lens cleaner disc etc.
Could be a hardware or circuit failure, such a lens/tray motor, faulty connection on the disc tray assembly, or another faulty component on the circuit board...either way useless explaining how to rectify that to u ;).
If cleaning the lens and using appropriate clean discs fails, then its probably not worth fixing. it still may be a dirty lens as many CD lens have a raised plastic rim which can make some cleaning Cd's not do a good job, so try a few times.
If u insert the disc and you hear it start to spin and hear some further movement noises and then get a invalid message it implies the CD tray assembly and mechanical parts are working, which still can mean dust on the lens which a technician can clean...but its more common to have a faulty disc reader, basically the laser no longer fires out the lens, so it fails to read anything...still tries but never will again :P.
Normally any other mechanical faults or issues cause a error in the disc read process, so u hear the disc go in and before it can do anything it gives a error or invalid message, suggesting something is damaged..
You unit is having an alignment issue. While most laser read consumer electronics are not build with end user adjustments in mind, you should go to the manufacturers website and see if there is a FAQ on the invalid disc error message.
Another option is to Google Brand Name DVD player invalid disc error, where Brand Name is the name on your unit.
If others are having similar issues, you'll find them and a possible solution.
I've had great luck with this method.