How can I collect my unclaimed tax refund

Please explain what you mean by "unclaimed tax refund."   What tax year (s) are you asking about?  Did you file tax returns for those years?   Were the returns accepted or were they rejected?

The simple answer is: you file a tax return (form 1040 plus any required additional IRS forms). There is a three year limit for filing for a  refund (there is no time limit if you owe money). Example: to claim a 2021 refund, you must file by April 15, 2025 (3 years from April 15 2022, the deadline for filing a 2021 return). If you filed for an extension, the deadline is Oct. 15, 2025. It is too late to claim a refund for 2020 or earlier years (unless you filed for a 2020 extension). 

But, as others have said, it depends on the details of your situation.