PS Plus fell over completely on the 4th of January 2016, and users were naturally less than happy. At the time Sony PlayStation said that they would be making it up to their customers and they've come good on that two weeks later.
Every single person on the planet who had an active PS Plus account on the 4th of January will get one extra day on the service for free. You should be getting an email about it this week with a voucher code - just apply it and you'll get the extension at the end of your current PS Plus period. The voucher code expires on the 31st of March 2016 so be sure to use it before then.
It's hardly the most generous apology but it is better than nothing and does at least restore the extra time to everyone who might have lost it. Now how will you make the most of those extra 24 hours, if you even remember that you have it.