I have ubuntu 14.04.1 running with 3.13.0-93-generic kernel after a recent upgrade. UEFI is enabled from the begining. However I can see that some modules (e.g. virtualbox, DKMS) are not working and asking for disabling UEFI. My question is, if I disable UEFI from BIOS what will happen to the booting? Will is boot or not? It is a single OS system. Can someone explain exactly what will happen? I can't just disable the UEFI before knowing the consequences as it is a production system. Please help.
The problem is not the boot mode (BIOS/CSM/legacy vs. EFI/UEFI); it's Secure Boot. With Secure Boot active, Ubuntu 16.04 requires that all kernel modules be cryptographically signed; however, third-party kernel modules, such as those for VirtualBox, are not signed in this way. Thus, Secure Boot prevents their being loaded.
Note that Secure Boot is an optional feature of UEFI, so you won't run into this problem with a BIOS-mode boot. That said, switching from an EFI-mode boot to a BIOS-mode boot to get around this problem is overkill and is likely to lead to follow-on problems.
Fortunately, it's possible to disable Secure Boot. The details of how to do this vary greatly from one computer to another, so I can't write a simple description of how to do this that's universally applicable. This page of mine, though, presents several examples of how to disable Secure Boot, so you may want to check it out.