Recently I had to (re)shift certain plant pots (kept in a shaded area from a while (an year or so)) to a bright area. Earlier the plants leaves had turned dark green in shade probably due to their high chlorophyll content. Due to some reasons I had to shift the plants in a bright sunshine lit area again and white and yellow patches along with de-colour-a-tion patches are appearing on the leaves more so in Aloe Vera, Stone-Apple and Neem plants. (what is the reason to this?) Also new leaves have stopped appearing. What deficiency sign is plant showing. It may not be underwatering, as if i over water plants I see complete yellowing and falling of leaves. Will I be loosing plants as summers appear, since it's only winter here. What can be done ?
Plants get used to their environment in which if sudden changes are made they can go into shock. This could result sudden drop of leaves, etc. Such change is what you have described, moving from a shaded, indirectly lit area to direct sun. Sometimes, based on the conditions and the type of the plant, they can acclimatize slowly in these cases as well and start growing again.
To avoid problems changes in the environment should be made gradually, so the plants will have time to slowly acclimatize to the new environment, avoiding shock, drop of leaves, etc. This means for example that you should have moved the plants closer to a window first, then putting them onto the windowsill.
Note that certain plants will rarely do well in direct sun conditions. For example Aloe Vera leaves will start yellowing due to the sun stress, so you should keep it in indirect bright light conditions instead.
Here is a sun stressed Aloe Vera, which possibly would slowly wither away.