Not only are YOU missing something — the IRS is missing your return! They cash any checks immediately, so it is safe to assume that the mail went astray.
That’s why you should always send mail to the IRS as a registered letter, with return receipt requested.
Did you keep a copy of that tax return? If so, refile — and send registered — along with a new check.
Ask the bank to put a stop payment order on the original check, and copy their acknowledgement. Then send a COPY of the original check AND the stop payment confirmation, along with your new check, and refile the return.
If you want help on this go to IRS.gov and search for form 911 –to catch the attention of the taxpayer assistance office.