You need to contact the organization that issued your 1099-G for a replacement. If you have an online account, you can download it from there.
What was it for? Unemployment or a state tax refund or something else?
If you got unemployment but didn't get a 1099G
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-report-unemployment-if-i-didn-t-get-a-1099-g/00/26866