Firstly, did you get your passport and visa stolen? Or just lost them?
If it is stolen, you must report it to the local police and obtain a police report. You will need this to apply for a new passport and replacement visa.
If it is either stolen or lost,
you must contact your embassy to obtain a temporary passport to return to the UK. Each embassy will guide you their way to get your new passport. Then you have to apply for permission to return to the UK, though, the application system varies depending on your visa form – is it vignette attached to your lost passport? Or is it BRP (Biometric Resident Permit)?
If it is a vignette attached to your passport and lost together with your passport,
you have to re-apply for Tier 4 General visa paying the full application fee. You do NOT need CAS letter, though you have to prove that you are a student (e.g. original university letter) hence you may be required to contact your university. Once you get the new enter clearance to return to the UK, you now have to apply for BRP (ref: http://www.beaumontparkassociates.com/#!Summary-of-Changes-Made-in-Tier-4-in-July-2015/c13p1/icw5rdnr175)
If it is BRP, you must contact the Home Office immediately by emailing BRPLost@homeoffice.gsi.gov.uk, calling +44 (0) 208 527 3263, or via online report (https://www.gov.uk/biometric-residence-permits/lost-stolen-damaged）Your BRP card will be cancelled, and you must apply for a single entry visa to re-enter the UK. BRP cannot be applied from outside the UK, therefore once you entered the UK you will need to apply for a replacement BRP. (Please see the question “I lost my BRP, what should I do?”)
BPA assists your re-application to enter the UK from outside and replacement of BRP. Please do not hesitate to contact us.