To clarify the previous answers:
You probably received the CP2000 which assessed an increase in tax for 2017. The IRS assumes that HSA distributions (which would have been reported to you on form 1099-SA) are taxable, until you indicate on form 8889 that you spent the entire amount on qualified medical expenses. So if the IRS has a copy of an 1099-SA that you didn't report (presumably because, as Critter suggests, you did not include a form 8889 with your return), then you will have to file form 8889 to show that all the distribution was for qualified medical expenses.
So do the following:
1. Do what Critter and rjs say: go look at your HSA custodian's website to see if there were any distributions in 2017 (note that forms 1099-SA are normally made available by HSA custodians on their website; if not, then call them).
2. If you don't find a 1099-SA for 2017, look at your transactions for 2017. Was anything paid out at all from your HSA? Note that account maintenance fees (if any) should not count as distributions - if the IRS thinks that you should have reported what were the account maintenance fees, then the HSA custodian made a mistake in its report to the IRS.
3. Your HSA custodian is the financial institution that handles your HSA. This is the same company that gave you the debit card with which you can pay for qualified medical expenses. Worst case, ask your employer.
4. Make sure that you respond in a timely manner to the IRS - you generally have about three weeks to respond (but check the letter for the deadline). Do this even if you don't have an answer from the HSA custodian yet; just tell the IRS that you are double-checking with the HSA custodian.
5. How you will resolve this will depend on what happened:
5.A If your HSA custodian generated a form 1099-SA and you never got it, get a copy and reply to the IRS (note: as Critter says, you will probably have to add a form 8889 to your return, but look closely at the letter's instructions because the IRS will probably not want an amended return, just the form).
5.B If you are sure that you had no distributions from your HSA in 2017, see if you can get a letter stating that from the HSA custodian which you can forward to the IRS. Failing that, reply to the IRS that your HSA custodian is (name/address/federal tax ID number) and that your account shows no distributions for 2017. It may be that there was a clerical error, and that the IRS thinks that your custodian is some other institution entirely.
