I wandered in to Understone Keep to sell some stuff off to the blacksmith and wizard, but a couple of the NPCs in that place started attacking me for no reason. The Thalmor guys, Jarl Igmund, as well as Igmund's housecarl and steward are all attacking me for some reason, and I can't get them to stop. None of the guards or other NPCs attack me or do anything, they just wander around doing their "omg its a fight" thing.

What do?

Fight back!

pull an ulfric and usurp a throne?

Did you kill someone you weren't intending to? Are you following any dark paths with a brotherhood of murderers? Reload the save and make sure it's not a glitch or some other error. Pretty sure the Jarls are essential characters so you'd only be able to down him/her.

I did the forsworn conspiracy quest , it didn't feel right to help this king in rags guy escape so I killed him and took the key to escape on my own.One of the silver-bloods met me at the exit of the escape path , did some speech about doing something heroic blah blah then gave me a family smithing ring.But now on this character the guards just attack me on sight and when i try the option to pay o go to jail they just go into a loop and keep re-arresting me.This is after being a stormcloak and changing the guards to stormcloak soldiers etc.So for me going back enough saves to negate whatever bugged this would mean literally days of replaying.

Maybe you've done this quest I'm not sure but this is the same situation I'm in with markarth atm.it was suggested to go back to the shrine where the guards tried framing me and they might be spawned and alive again and they qould arrest me or let me pay the fine , but they're dead on the floor like I made them defending myself.I had no choice in any of this even at that part of the encounter when I chose not to fight guards hoping there would be some other option the guards just attacked me without saying anything else.

Either Markarth is badly broken or I'm simply making a huge misstep here , I'm now playing through and saving frequently when I go anywhere near Markarth on my other characters.

I had Markarth glitch out on me once. I FT'ed into the city and everybody was red and I was immediately attacked by the guards. I loaded a previous save and FT'ed to a location outside the city then walked to Markarth. The guards out front ignored me and when I entered the city through the gates, everything was fine.