is frozen hamburger ok to eat after the sell by date?
Assuming you didn't buy it frozen, the sell by date is for when it's just in the fridge. As long as you froze it before that date, and you use it immediately after thawing, it'll be safe to eat. If it was already frozen when you bought it, the sell by date is just for quality, not for safety.

In either case, if it wasn't wrapped tightly it may have lost some quality. Things do slowly dry out in the freezer if the packaging or container isn't airtight, so the texture can deteriorate, and they can also take on odors from the rest of the freezer.