Your only choice for a 2018 return is to mail it in ...
To finish a prior year return started in TT online:https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb
If you already paid for your online software you can ask customer support for a download of the desktop software.
If you didn't already pay for your 2018 product prior to the shutdown, you can either purchase directly from the TurboTax website as mentioned above, or you may find the product at a discount as a download at Amazon, etc. CAUTION: There are a lot of scam sites, so do not try to purchase from any "too-good-to-be-true" discount site on the Internet that you find in a search. You might end up with pirated software, ID theft, malware, etc. Only a few large well-known sites like Amazon, NewEgg, Staples, Office Depot, etc., are authorized to sell TurboTax downloads.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.
If you are getting a refund, there is no penalty for filing late. If you owe tax due, then file and pay the amount due as shown on the Form 1040, but expect a bill later from the IRS for the penalty and interest you will owe. Only the IRS will calculate this—TurboTax will not calculate it.
If you want to just get the forms and prepare them by hand, that would be free--except for postage to mail them.
Here is the IRS 2018 Form 1040
And instructions https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf