An immobilized walker cannot pivot (p.74)--so while overwatch is still allowed (p.84)--you are limited precisely moving targets within the 45 degree horizontal and vertical arc present at the time of immobilization.
Despite the base contact, the tank is not locked-in combat (p.76). This means the tank is a valid target during the shooting phase on your turn (from both the walker and other shooters), also provided that your immobilized walker can target the tank by being in it's current, unchangable 45 degree shooting arc.
The walker can engage in the assault phase with the tank right after being rammed (p.84). However, because it is immobilized, it would have one less attack to a minimum of one attack (p.84). This reduction will apply to the subsequent assault if you remain btb.
7th edition seems to keep almost identical text regarding walkers, ramming, and overwatch.
If my walker isn't immobilized, what changes?
Lots would change if your enemy wasn't a tank, but since it is a tank, the differences are fewer: if you were not immobilized, you could 1) walk away, 2) overwatch in all directions, 3) have all available assault attacks and 4) pivot and shoot at anybody within range.