A mobile hairdresser said it was "really distressing" having to close her business due to an "admin error".
Julie Price, from Flintshire, said she failed to qualify for the Self-Employed Income Support Scheme (SEISS) during the pandemic after accidentally failing to submit her 2018-2019 tax return.
She had to get a new job as an estate agent after taking family "handouts".
HM Revenue and Customs (HMRC), which runs the SEISS, said it would speak to Mrs Price "to clarify the situation".
The SEISS is worth up to 80% of trading profits, averaged over three months, up to £7,500 in total.
2.7 million people have been helped by it and the chancellor announced an extension of it in the Budget on Wednesday.
Mrs Price has been a business owner for 10 years and said she filled the return in online but did not press "submit" to complete the process.
Despite being able to prove that she had paid the tax that was due, she said HMRC would still not allow her to receive the support.
"Had we not had family that stepped in and helped us I dread to think where we would be," said the 42-year-old.
"We were lucky with that, but on another side we'd always lived very comfortably and very well and to all of a sudden have no income whatsoever, it was a massive shock and very upsetting and caused us lots of sleepless nights and stress."
Mrs Price said she paid her taxes as a self-employed hairdresser "on time every year for 10 years" but because of an "admin error" she had effectively been told "sorry, tough luck".
She added: "I got to that point where it was getting that I was asking for handouts off my parents a lot and I didn't know how this was going to go on for."
Getting an estate agency job has given her "security" again, but she said closing her business was painful.
"It has caused me great distress and upset to actually lose something that I've spent 10 years building.
"And those customers aren't just customers to me, they're friends and I hate the thought of having to let anybody down, it's been really distressing."
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