Koba:
You need to contact USPS to determine if the return was received by the IRS. If so, then contact the IRS.
If they have no record of receiving the return, ask if you should submit a second return. They may tell you to wait a bit, or go ahead and submit the second copy. FOLLOW the IRS instructions, because submitting a SECOND return will only cause confusion if the FIRST return is just being processed late.
I assume that you had a refund coming. If so, then just submit the second tax return.
If payment was made, has the check cleared?? If not, you can probably assume that the first tax return was lost. If the check DID clear, then get a photocopy of the cleared check and enclose it with the second return to show that you indeed paid the tax before the 15 April 2008 filing deadline.