And you filed it on a 2018 tax return form 1040? And you mailed it? You had to use the 2018 Desktop program (not the Online version). Then only the IRS can tell you. When did you mail it? This year the IRS is taking longer to process mailed returns because of Covid-19.
Explain what you did to file that 2018 W-2. The only correct way would have been to use 2018 tax forms and mail it with the W-2 attached. Is that what you did? If so, then mailed returns are taking a very long time for the IRS to process and you will have to just be patient.
Or did you make the mistake of using the online program and e-filing it? If you did that, you really filed a very incorrect 2019 return because online was only for 2019 returns. If you incorrectly used the 2019 software then the IRS may have spotted the mismatch between the income reported by your employer for 2018 and for 2019 and you may end up getting a letter. Or the return might be in "review" and take a long time before you even get a letter.
Tell us what you did to file the return and we can be more helpful.