Work through these steps for Troubleshooting Windows 7 Failure to Start.
Since you have no disk drive, there are bootable flash stick alternatives for most steps. Using Hiren's BootCD From USB Flash Drive (Pen Drive) y
ou can check the HD using maker's diagnostics and run Disk Check
from bootable XP explorer. That will confirm the HD condition which needs to be done first.
If those don't work Hiren's also has bootable Partition Wizard Check File System
and Partition Wizard Partition surface test
to check HD condition.
Once HD is confirmed, Hirens also has bootable AV to check for infection, then you can check the Active flag is on System Reserved partition or C using Partition Wizard and then run Partition Wizard Rebuild MBR Video Help.
If that doesn't start Windows 7 boot into the Windows 7 installer provided in Step 5 of Troubleshooting steps along with tool to write to flash stick, to run Startup Repair - Run up to 3 Separate Times
So as you can see, almost every step in the tutorial which offers everything possible to repair unbootable Windows 7 can be run from bootable Flash stick, up to copying out your files using XP explorer to run Factory Recovery from its partition on the HD, or getting the superior Clean Reinstall - Factory OEM Windows 7