You have a couple of options.
1. You could go now with your still valid PR card. When you enter Canada, you can tell the immigration officers that you have been taking care of your sick father for the past 4 years but that he is better now. Then one of two things may happen.
1a. Immigration officers let you in without reporting you for not meeting the residency obligation. You can then stay in Canada for 2 years straight and that will put your PR status back in good standing and you can apply to renew your PR card.
1b. Immigration officers report you for not meeting the residency obligation. You will have to appeal for your PR and prove your fathers situation. If you win your appeal, you may keep your PR status and apply for a new PR card. If you lose your appeal, you will have to leave.
2. If you don't return before your PR card expires, you will need to apply for a PR travel document proving that you were taking care of your ill father. You can find the application forms here: http://www.cic.gc.ca/english/information/applications/travel.asp Apply to the embassy in New Delhi. Send as much evidence as you can, statement from your father, doctors reports and other medical reports. A statement from his doctor might help too. After applying for the travel document, one of two things may happen:
2a. Immigration agrees that you had good reason not to meet the residency obligation and approves your travel document. You can return to Canada and apply to renew your PR card.
2b. Immigration doesn't agree with you and refuses your travel document. You can appeal within 60 days. If you don't appeal or lose the appeal, you will lose your PR.
If you lose your PR through 1b. or 2b. you can of course apply for PR again if you qualify.
