Verify that you've *definitely* got the booking details right.
If the booking reference contains a letter I, are you sure it's a letter I and not a number 1 ? If you entered a credit card number to access the booking, are you sure it matches that used to make the booking ? Are you sure the to and from airport codes are correct ? Are you sure you have the right date ?
Assuming you're sure it's all correct, turn off your PC and then turn it on again - occasionally Internet Explorer has issues with the Ryanair website when the PC hasn't had a reboot for more than a week (putting a PC into hibernate doesn't count as a reboot)
If that doesn't work, wait 30 mins, and try the Ryanair website again. Verify that if you make a dummy booking for a new flight (don't enter your credit card details), you can see a price as to how much it'll cost - this is to confirm that you can get a response from the reservation system.
Do Independent Holidays, through whom you presumably booked your flight, perhaps have an out of hours emergency telephone number which you can call ?
If that still doesn't let you do a checkin online, then your best bet is to turn up at the airport at least 2 hours in advance, with a credit / debit card and the email you originally received confirming your booking, and possibly pay the extra fee for airport check-in / lost boarding card
Good luck !
Last edited by davidjohnson6; 24th Jun 2012 at 20:35.