If your card is lost or stolen
If your card is lost or stolen, you must report this to us as soon as possible by phoning 0300 123 2412 from the United Kingdom or (+44) 20 8527 3623 from abroad. You must then apply for a replacement card using application form ICFN(RC) - if you do not do this within three months of reporting the loss or theft, you may have to pay a financial penalty of up to £1,000 or we may shorten your permission to stay. You can download form ICFN(RC) and the related guidance from the right side of this page.
http://www.ukba.homeoffice.gov.uk/while-in-uk/do-i-need-id-card/responsibilities/#header6
如果你回国申请的话,那就是重新准备材料申请出国了