can you watch videos from other online sources e.g. youtube?
If not, then the DVD & iplayer problems are probably linked.
Assuming a windows system, then Control panel>system>device manager would be the first port of call. Have a look to see if something is reported as a problem. Running the add new hardware utility might be all that you need to fix the problem.
Run the windows update thingy - click "custom install" & look in the non-essential updates folders as well, for driver & software updates.
Next port of call would be to check the cables inside the PC. Also reseat any memory modules. If you have a separate graphics card then reseat that as well.