The mission team assumes that, due to the huge dust storm that already covers a quarter of the surface of Mars, including the location where the rover is, and that does not get sunlight to the surface, the battery charge has dropped below 24 volts. Under these conditions the solar panels can not recharge the batteries, which has probably caused Opportunity to go into safe mode.
In these circumstances, all on-board systems are turned off except the minimum to prevent their temperature from falling below the limits of what their components can support, and a clock that will check the load levels every day at 11:00 local time.
If they are sufficient, it will try to start the necessary systems to get in direct contact with the control of the mission using the low gain antenna, which allows very slow communications but does not depend on any orbiter acting as a communications relay. If not, it will reset Opportunity to rest. There is a fairly detailed description of the procedure in Mars Exploration Rovers Surface Fault Protection.
Given the force of the storm in the JPL say that it will be several days before the solar panels of Opportunity can return to give enough charge to the batteries. The question is whether this will happen before the rover freezes.
But whatever happens, do not forget that Opportunity was designed to last 90 days and has been in service for more than 15 years.