What is TOGA LK in Airbus 320? How to come out of TOGA LK?
TOGA LK (TOGA thrust locked) is displayed on the FMA after an activation of the ALPHA FLOOR protection when the activation conditions are no longer fulfilled (A FLOOR is displayed during the ALPHA FLOOR condition). The idea is that the thrust does not automatically reduce again after ALPHA FLOOR because this could result in a situation where the speed decreases again. The procedure to get out of TOGA LK is described in the FCTM (Flight Crew Training Manual):

ALPHA FLOOR
When the aircraft’s angle-of-attack goes beyond the ALPHA FLOOR threshold, this
means that the aircraft has decelerated significantly (below ALPHA PROT speed):
A/THR activates automatically and orders TOGA thrust, regardless of the thrust lever
position.
The example below illustrates that:

The aircraft is in descent with the thrust levers manually set to IDLE.
The aircraft decelerates, during manual flight with the FD off, as indicated on the FMA.


When the speed decreases, so that the angle-of-attack reaches the ALPHA FLOOR
threshold, A/THR activates and orders TOGA thrust, despite the fact that the thrust
levers are at IDLE.
When the aircraft accelerates again, the angle-of-attack drops below the ALPHA
FLOOR threshold. TOGA thrust is maintained or locked. This enables the flight crew
to reduce thrust, as necessary. TOGA LK appears on the FMA to indicate that TOGA
thrust is locked. The desired thrust can only be recovered by setting A/THR to off,
with the instinctive disconnect pushbutton.
ALPHA floor is available, when the flight controls are in NORMAL LAW, from liftoff
to 100 ft RA at landing. It is inhibited in some cases of engine failure.

(Airbus A320 FCTM - Operational Philosophy - AP / FD / ATHR, emphasis mine)
The TOGA LK state is not necessarily linked to a Windshear, but since ALPHA FLOOR is part of the Windshear escape strategy, it can certainly appear in this context. The FCTM describes the following strategy to escape a Windshear:

There are three efficient tools to assist the flight crew to escape:

The alpha floor protection
the SRS AP/FD pitch law
The high angle of attack protection

When the alpha floor protection is triggered, the A/THR triggers TOGA on all
engines. The FMA displays A FLOOR, that changes to TOGA LK, when the
aircraft angle-of-attack has decreased. TOGA/LK can only be deselected by
turning the A/THR off.
The SRS pitch mode ensures the best aircraft climb performance. Therefore, the
procedure requests following the SRS pitch bar and possibly full aft stick, in order
to follow the SRS orders and minimize the loss of height.
The high angle-of-attack protection enables the PF to safely pull full aft stick, if
needed, in order to follow the SRS pitch order, or to rapidly counteract a down
movement. This provides maximum lift and minimum drag, by automatically
retracting the speed brakes, if they are extended.

(A320 FCTM - Supplementary Information - Adverse Weather, emphasis mine)