my friend uses lync 2010 in his company. earlier i used to chat with him through my skype , but now i cannot . i see him offline and he sees me as presence unknown in his lync

can you tell me how can i connect with him as before..through my skype

If you cannot connect with him, it may be that federation is broken on his end (assuming you mean Skype consumer, not Skype for Business on your end).  You should both delete each other's contacts in your clients and then try to re-add and re-establish the connection.  He should also test with other Skype users to see if it's issue with his deployment.