I was struck by the spell Sepia Snake Sigil and now I am Immobilized. What does this do, can I cast spell? Can I use abilities etc?
As Hey I can Chan pointed out in the Rules compendium p.35
"An immobilized creature can’t move out of the space it was in when it became immobilized. It otherwise functions normally unless it’s flying. Immobilized fl ying creatures that have the ability to hover can maintain their initial altitude. All other flying creatures subjected to this condition descend at a rate of 20 feet per round until they reach the ground, taking no falling damage."
However it may not be the answer you are looking for.
You should put closer attention to the first part of spell effect.
Sepia Snake Sigil:
If the target fails its save, it is engulfed in a shimmering amber field of force[...].
While trapped in the amber field of force, the subject does not age, breathe, grow hungry, sleep, or regain spells. It is preserved in a state of suspended animation, unaware of its surroundings. It can be damaged by outside forces (and perhaps even killed), since the field provides no protection against physical injury. However, a dying subject does not lose hit points or become stable until the spell ends.
Basically you are like an insect, trapped inside an amber.