You can return the money to your HSA in some cases, you'll have to talk to your HSA administrator about how to go about it, but the IRS allows you to return errant HSA distributions:
A-37. If there is clear and convincing evidence that amounts were
distributed from an HSA because of a mistake of fact due to reasonable
cause, the account beneficiary may repay the mistaken distribution no
later than April 15 following the first year the account beneficiary
knew or should have known the distribution was a mistake. Under these
circumstances, the distribution is not included in gross income under
section 223(f)(2), or subject to the 10 percent additional tax under
section 223(f)(4), and the repayment is not subject to the excise tax
on excess contributions under section 4973(a)(5). But see Q&A 76 on
the trustee’s or custodian’s obligation to accept a return of mistaken
That is out-dated, as the penalty is now 20%, but the rest should still be accurate.
Source: Health Savings Accounts—Additional Qs&As
If you spent money on something that clearly wasn't a qualified expense (pizza?) you'd owe a 20% penalty, follow the instructions on Form 8889 for that.