There are any number of reasons why you might be having trouble and we will need more info to be able to sort it out. Please note:
Any new hardware/software that you recently put in.
How long you have had the problem and whether it was working properly before.
Is this a new build or brand name computer or one you've used for a while?
What troubleshooting you have already done.
Some idea of how much you already know about working with computers so we have an idea of how complex the suggestions can be.
Any other info that might help us understand what is going on.