Loss of power, very low power on reverse

[cashville]

Hummer H2 2003 - 65k miles

My brother did a 40mile journey, he stopped for a few hours, then when getting back, after about 30 miles, he noticed loss of power on the H2, and on the dashboard it showed Transmission Hot. He stopped for some minutes, then turned the engine on, and nothing showed on the dash, he made the 10miles to get home, and parked the H2.
3 days later, I went to check on it because I was on vacation, I started the car and noticed a loss of power in Drive, just a little. I tried to reverse, and the reverse barely worked. I left it parked and didn't drive it any further.
What do you think it could be? I've read a little on transmissions going out, but I didn't find anyone complaining about reverse barely working...