It's not uncommon for cheaper and/or neglected recess trays to jam over time as the joint between tray and frame becomes blocked with detritus, weeds and god-knows what else. In some cases, natural oxidation of the often-cheaply galvanised steel forms a weak bond between the two components, and on really cheap trays, the galvanising may fail completely and allow a rust bridge to form between the exposed steel of the tray and frame. That's part of the reason why we recommend trays are lifted out at least once a year and the edges of both tray and frame lightly smeared with lubricating grease before replacing. This helps limit the oxidation, but also ensures the tray is much easier to remove when necessary.
When a tray is jammed, there are several techniques that can be used to free it from the frame. The simplest starts with using compressed air or a high-powered water jet to blow out any detritus trapped between frame and tray. While this method works well for simple detritus problems, when there's a rust problem, squirting a penetrating oil (such as WD40) into the joint between tray and frame can sometimes free-up the tray sufficiently to allow it to be lifted clear. However, it may take two or more applications with, say 12-24 hours between applications to allow the oil to fully penetrate the joint.
There are manhole cover lifting devices that are much better at raising jammed or heavy covers that the brute force of a man on his own. These gadgets can be hired, if necessary and work well in most cases.
Occasionally, we come across trays that are well and truly jammed, and the only practical option is to remove the paving from the tray, and then use an angle grinder to cut out the tray. When it comes to re-covering the chamber, a whole new frame and tray will need to be installed, which is why this is the 'last resort' option.