Hi sorry to take over this post but my turtle is in a similar situation and I'm freaking out a little because he seems to be dead but I'm in denial about it.
It's gotten pretty cold around here lately but I have a thermometer and I've noticed the temperature never falls below 68*F. I'll still be sure to bring it up at different points throughout the night though. He basks every morning around 9 to 12 which is when the most sunlight falls on his tank. The rest of the day he likes to swim around and sleep on rocks and on his basking ramp (underwater). He was perfectly normal yesterday.
This morning i noticed that he was in the same position as he was for several hours last night - simply resting at the bottom of the middle of the tank. I'd poked his limbs several times and they'd reacted so I thought he'd just decided to sleep there. I was concerned this morning though because he wasn't basking, so I picked him up and he felt a lot heavier/denser(?) and did not react. Usually it takes a couple seconds before he starts kicking but nothing happened this time.
His eyes are half open and they don't react to anything. I jiggled him slightly and his head shook around limply but when I touch his limbs they pull back immediately and then slowly come back out after a few minutes. I am always in huge denial about sad things so the fact that his limbs are still reacting is giving me slight hope...

He's been doing pretty well lately so I don't see any reason for him to suddenly kick the bucket
Other information:
He is probably a couple months over a year old, weighs 10 grams and is 1.5 inches.
He usually has a very small appetite, but due to some changes, has barely eaten in the past week.
About a month ago I noticed he was gasping when I took him out of his water, and there were slight bubbles. However, I watched him closely for the next couple of weeks, and I never saw him gasp or see bubbles form again. That was when I bought my filter and upgraded to an appropriately sized tank.
He had soft shell a couple months after I first got him, so I invested in a basking ramp and UVB light. Since then only the 3ish back scutes are soft.