Step 1
Maintain use of the bag until breathing is normal.
Step 2
Once breathing is under control, find your favourite chair to relax in. Fix a drink of your choice; either a cuppa tea or something slightly stronger. Then relax.
Step 3
Once comfortable, think of the following: as Guya has pointed out, there are, or will be in the next year, possibly 1 million people who will share your OMG (Oh my God) moment. You are not alone.
First plan, don't think about an immediate resolution to the problem. You will be spending some time on this, and decisions shouldn't be taken immediately.
Step 4
You'll have to start somewhere, so possibly the first thing to do is have a look at your situation. Are there any other accounts in either your name, or joint? Have you had any other income, earned or unearned, in the last 5 years? Do you know if you are above the basic reporting levels for either filing a tax return or the FBAR for the last 6 years?
Step 5
Once you've settled down (and it may take a few days), you're going to have to do some reading. Google is your friend, and there's a lot of information out there. Not all of it is reliable, so don't jump on the first bit of information you come across. Look for articles on 'quiet disclosure' or the 'Streamlined program'. As Guya mentioned, there is an official IRS program for those in your situation. There are about 6 different ways of approaching this, starting with doing nothing (which is becoming a very difficult and financially unsafe choice). The most 'official' (the one the IRS would prefer you to use) is the streamlined program.
Step 6
Once you have an idea of what's involved, you'll have to make your first choice. Are you capable of handling this situation on your own? Many do, whichever method they choose to utilise. It's not insurmountable.
Or, if you're still feeling overwhelmed by the whole thing, perhaps it's best to enlist professional assistance. There are smaller firms in London who specialise in US/UK tax affairs who can help you. There will be a cost.
Step 7
If you were aware of your responsibilities and wilfully choose not to file, there is a programme available. If you were not aware of your responsibilities to file, then whatever you do, do not allow anyone to talk you into the OVDP program.
Come back to this site if you like when questions arise. Only Guya or one of the other professionals are qualified to make intelligent comments, but the rest of us may be able to assist you as to where to look for an answer to your questions.