I just changed my alternator and battery now i have no power in my H3 Hummer. I can even get it to go out of park and my key is stuck in it.

Did you clean and tighten the battery connections? Did you accidentally reverse polarity?

Check all the battery connections and harness to alternator, then check you didn't blow a mega fuse or fusible link, if you have a multimeter, check of continuity or opens along the charging circuit. In the meantime you can mechanically release the park shifter so you can release your keys. Check your owner's manual for a how-to.

How do i check the mega fuse? I don't know what fuse that is. As far as checking continuity, i dont know much about that or how to go about it.

Look in your power distribution box under the hood, a fuse tester or a meter set to ohms can check it. Check "continuity" by setting meter to ohms, you want to see zero ohms or near to it, it means the wire is still intact. An "open" means you have a break in the wire (ohms will read "infinity"). The fusible link goes from the alternator to the junction box (by the PS pump), it tends to get very hot, if it 'opens' then you get no power.