Not the easiest thing to do in the middle of winter (and also not the best timing, either). Soak the entire rootball in a big tub of water for about 30-45 minutes. Then you should be able to loosen it with a strong stream of water. Or sluice it through the water and use your fingers to tease it apart.

If you are unable to do that, then I'd suggest root pruning and a manual stimulation or even slicing through sections of the solid rootball.

a plant id might help ...

i cant imagine how big a new pot you would need to spread out the roots .... yikes ...

i had a spider plant like this once ... and since i had many others ... i just cut quarters out of the root mass .. leaving enough of an 'X' so that when i fit it right back into the same pot.. it would remain at the same level ... and then i jammed new media into the open spots ... the spidey never cared a wit .. and within a year.. had refilled it again ...

since i had many others .... i ended up just getting rid of it.. and replanting the pot some some fresh new babes ...

whatever you do.. dont forget.. the mot important thing in a pot.. is the media.. use some good stuff ...

if you dont know what the plant is... give us some pix of the top ...

and if you can propagate the plant .... keep that in mind for a long term resolution of the problem ...

ken

Where are you located? These kind of tasks are best done in Summer
assuming that you are in colder climate and in Summer the plant is
situated outdoors.

First do not leave it out like that. At the least wrap it with many layers of moist newspaper to keep it hydrated. You cannot avoid pruning roots. In summer you can be quite aggressive. For now I would suggest first to slice the bottom inch using a knife or a pruner. Sort of like this:

You have to do the above otherwise it is very difficult to do the rest.

Then take a chopstick or a screwdriver and "comb" the sides vertically to loosen the mat of roots. You will have to prune a bit here and there. Keep the roots wet either by spraying or dunking in water every now and then. It may look something like this:

I was doing this a bit out of season and so in the above I did not remove all the soil. But made sure all the roots on the sides are free. And pulled out the free ends of the roots (roots that are detached because of pruning) from the soil.

Then you can pot it up. Needless to say have your soil and pot ready before all doing any of the above.

I work in a greenhouse and that doesn’t look bad to me. I’d take a paring knife and simply slice down on the outside of the roots where they were pressed against the pot - vertically from the top to the bottom - three times, evenly spaced around the pot. The cut should be an inch or so deep. The newly-cut roots will quickly grow outwards into their new larger space.

I do this all the time when I buy marked-down perennials in July, as by that time they’re rootbound. I never have any problems.

stick it back in the pot or one of the same size and wait till June.

take it outside then and spray the roots using the jet setting on the hose. That’ll loosen them up and get out bits of soil. Trim off long circling roots.

always keep the roots moist when working with them outside of the pot. If you let them dry out, they’ll die. The smallest feeder roots are the first to go.