We've just had the announcement that there will be no GCSE or A-level summer exams in 2020 - latest Government announcement here (opens in a new window). I'm told that this is the first time this has happened since the exams system was instigated in 1888, so we really are in uncharted territory. It's come as a huge shock and a great disappointment to a lot of people, but it may not be as bad as you think.
Instead of sitting exams, candidates are to be awarded a grade "which fairly reflects the work they have put in" based on their teachers' assessments, taking into account the results of mock exams and other assessments. The exam boards will then process these assessed grades and come up with a final grade, so the grade you are given may not quite match what your teacher has submitted.
The announcement says that the aim is to provide these calculated grades to students before the end of July. Teachers will have to be given guidelines to work to, and then time to process the information they hold and submit their grades; we don't know what the timescales are but my guess is that the deadline for the teachers' submissions will be sometime around the end of May or early June - which is when you'd have been sitting the exams anyway.
If you think you deserve a higher grade than your past performance suggests then you could try speaking to your teacher and asking if there's anything you can do to demonstrate your improvement before the deadline. If you're lucky then they may give you an assessment to do in exam conditions at home (though that may prove too difficult to police). A few weeks' additional study, possibly with support from a tutor, might be enough to boost your performance, especially if it's been a while since you had a formal assessment in school/college. But don't just pester your teacher because you want a higher grade; teachers are under even more pressure than usual at the moment, and you need to be able to prove that you deserve it.
If that doesn't work out, and you are given a lower grade than you feel you deserved, then there will be an appeals process, and also you "will have the opportunity to sit an exam at the earliest reasonable opportunity, once schools are open again".
So you could even view this as a reprieve, especially if you feel you've left it a bit late before pulling your finger out and doing some serious exam preparation: you'll have an extra four or five months to prepare for an exam where you can truly demonstrate your ability, and in the meantime will be given a temporary grade as a stop-gap. And a lot of the other candidates will have done a lot less work than you over the summer!
If you're wary of using a tutor because you've never tried online tuition before then don't be afraid to give it a go. There are many tutors who only work online, and most of the rest of us have adapted quickly to the current circumstances and are also able to offer an online service, using video conferencing, screen sharing and online whiteboards. You can find instructions for BitPaper and Zoom, the software that I use, here - have a play and see how easy it is to get to grips with. Of course you also have the option of going it alone - your school teacher will still be offering remote support as best they can - but a tutor will be able to give you more comprehensive peronalised support. If you pick someone local to you (and who isn't online-only) then you'll have the option of moving to face-to-face sessions once we're over the worst of the current crisis.