What type of plan is it? (There are a lot of plans that fall into section 125.) If you mean a health care flexible spending account, that is not reported anywhere on your W-2. Box 12 code W is only for HSAs, not FSAs. You need a corrected W-2. If you file without a corrected W-2 and just remove the code W entry, you can expect an IRS letter and tax assessment.
https://www.irs.gov/instructions/iw2w3
If you mean a dependent care flexible spending account, that would be reported in box 10, not box 12.
