Your HSA contribution limit is pro-rated for the number of months that you were HSA-eligible, those months that you were covered by the HDHP and not the other plan. Any contributions beyond that pro-rated limit are excess contributions subject to a 6% excess contribution penalty each year that they remain in the account unless you obtain a return of the excess contribution before the due date of your 2020 tax return. If you nothing and do not obtain a return of contribution before the due date of your 2020 tax return, it gets real expensive to correct the excess to avoid continuing 6% excess contribution penalties because not only will you owe income taxes and penalty on the excess contribution with your 2020 tax return, you'll also own income taxes again plus, if you are under age 65, a 20% penalty when you later take a normal taxable distribution to correct the excess. Note that only the amount of the excess can be returned before the due date of your 2020 tax return, not any amount beyond that.
Since the contributed funds were originally part of your pay, the return of excess contribution is paid to you, accompanied by any investment gains attributable to the excess contribution. The excess amount itself is taxable on your 2020 tax return and any investment gains that are distributed are taxable on your 2021 tax return.
If your spouse is the other individual covered under the HDHP plan and your spouse is not cover by any other plan, your spouse could make a contribution of the remainder of the family limit that you are not eligible to have contributed and, if your spouse was age 55 or over, a $1,000 catch-up contribution.
