This happened to me too, and I ended up cramming 8 modules of work into some ridiculously short space of time (about 2 weeks or something), motivated by sheer panic. Don't do this! I ended up worrying all the way to results day and it spoiled my summer.
I agree with what's been said before about having a total break for a few days, set rewards for good work and focussing on your goals.
I did the following stuff at different points over the 2 years of A-levels and it seemed to work:
<Long post warning, and apologies if I'm stating the obvious>
Stress can demotivate, so have a look at exactly what marks you need to get from each module to achieve the grades you want and write it down. It might be less than you think. Go through the syllabus and see what you're confident about - that might make the marks you still need to get seem easier to attain.
Try 'revision lite' for a few days, making it as simple and low-effort as possible. For example, try writing 1 A4 side of short answer questions for yourself based on facts you need to know, go and do something of fixed duration for about an hour (Such as getting washed and dressed, or having lunch, or a walk round wherever you live, and definately not going on the internet or TV), and then answer your questions. After that, write another set and repeat. Works well for things like Chemistry.
If it's something like maths, replace your own questions with short answer questions from the textbook.
Repeat this cycle from, say, when you get up till 5pm, then quit and do whatever you want until bedtime.
Obviously this sort of revision cycle only works if you're not in college that day.
It's not the most comprehensive revision method, but it might help you get back into working.
You could try starting the day with whatever is most daunting to get it out of the way. Alternatively, you could start the day with easy things to get you into the right frame of mind. Try both, see what works.
Set a realistic amount to do for the day and make sure you do it. If you achieve the goal once, you'll feel more motivated to do it again tomorrow. Only focus on what you've decided to do for the day rather than the whole big chunk of work you have to do in total.
Do your first bit of revision at the very start of the day. Once the first bit is over with, you will feel less stressed about not doing any work and you'll find it easier to do more later.
If you have a full day at college, have a quick lunch and do a small section of revision afterwards. At college there's usually nothing more interesting to do so you don't get distracted.
Don't do really interesting things in your breaks from study. A '5 minute MSN/video game/chatting to friends break' doesn't exist, it will become an hour. A 5 minute snack /going doing housework/petting the cat/wandering round the house break works quite well.
Speaking of snacks, if you get hungry while revising, eat in breaks from study. Don't stop working early to satisfy cravings, but don't ignore them either.
Most importantly, don't try all of this in one go.