Basic electric roller screens are normally driven by a PSC induction motor with two identical windings. One is fed by the 'up' wire via the up limit switch, the other is fed by the 'down' wire via the down limit switch. The capacitor is connected directly between the two windings, so whichever winding is not being made live with mains becomes the capacitor winding. Depending on which of the up or down feeds is made live, so the roles of main and capacitor winding reverse and with them the rotating magnetic field that gives the motor a definite direction.
The screen should never reverse during use - e.g. if while it is down, you energise the up wire, the motor runs in the up direction until the up limit switch opens and it stops. If it reverses when stalled, fails to start until tickled or lacks torque, this indicates that the capacitor is internally open-circuit or disconnected. Without it, the motor cannot generate torque at standstill and has no impetus to run in any particular direction irrespective of whether the up or down wire is live.
A similar behaviour would result if the up and down wires were made live at once, causing the capacitor to be shorted out except when one of the limit switches is open. This should never be allowed to occur but might result from a shorted control switch or wiring, and also if the capacitor is internally short-circuit. Again there will be no starting torque, but in this case the motor may draw excess current, hum loudly and get hot very quickly. When the screen hits one of the limits, normality is restored for a moment as one of the feeds is broken. The motor will reliably restart in the opposite direction and then immediately be subjected to the two-windings-live situation again as the switch recloses.
With the screen in mid-travel (so both limit switches are closed) I would check for shorts between the two inputs, which should not show continuity. The capacitor is the next suspect.
Note that because the motor may at present run in the wrong direction relative to which wire is live, the limit switches are ineffective and it may overrun and wind the screen the wrong way around the roller. The limit switches will then be the wrong way round when normal operation is restored, which could rip or stretch the screen if not spotted.