Who is Contact for Missing refund for 2018?

You should have investigated a 2018 refund long before now.    All of the filing status and refund status sites are only for 2019 now.      You can check the IRS transcript site for 2018 and if it is not there then probably is was never accepted which means that it was never filed at all.

https://www.irs.gov/individuals/get-transcript

See this TurboTax help article:

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online/01/26322

"How do I finish a prior-year return that I started in TurboTax Online?

If you started your 2016, 2017, or 2018 return in TurboTax Online but weren't able to file it before the shutdown, you can finish and paper-file it in the TurboTax download software for PC or Mac.

Start by purchasing and downloading the software and install it on your computer. If you're also filing a state, be sure to download and install the corresponding state program (PC instructions | Mac instructions).

1. Now sign in at TurboTax.com using the same login you used to prepare your prior-year return.
If you don't remember it, try account recovery. Still no luck? Then just start over in the TurboTax software you downloaded and skip the rest of these instructions.

2. Under Your tax returns & documents, select Download .tax file (not the PDF) for the prior year you want and save it to your computer.

3. Back in the CD/download software, select Find a Tax File, and then browse to the tax data file you downloaded in the previous step.

4. Select the file and click Open.

5 . You can now finish up in the software. Your return will need to be paper-filed, as e-filing prior years is no longer available."