I boot with liveUSB. But how do i fix now my MBR, so that i can re-install Ubuntu? (after installing ubuntu i get mostly this blinking cursor and black screen)

root@ubuntu:~# fdisk -l



Disk /dev/sda: 500.1 GB, 500107862016 bytes

255 heads, 63 sectors/track, 60801 cylinders

Units = cylinders of 16065 * 512 = 8225280 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x000638ad



   Device Boot      Start         End      Blocks   Id  System

/dev/sda1               1       59769   480092160   83  Linux

/dev/sda2           59769       60802     8291329    5  Extended

/dev/sda5           59769       60802     8291328   82  Linux swap / Solaris



Disk /dev/sdb: 2013 MB, 2013265920 bytes

58 heads, 58 sectors/track, 1168 cylinders

Units = cylinders of 3364 * 512 = 1722368 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x1965c912



   Device Boot      Start         End      Blocks   Id  System

/dev/sdb1   *           1        1169     1965056   83  Linux



root@ubuntu:~# fdisk /dev/sda



WARNING: DOS-compatible mode is deprecated. It's strongly recommended to

         switch off the mode (command 'c') and change display units to

         sectors (command 'u').



Command (m for help): p



Disk /dev/sda: 500.1 GB, 500107862016 bytes

255 heads, 63 sectors/track, 60801 cylinders

Units = cylinders of 16065 * 512 = 8225280 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x000638ad



   Device Boot      Start         End      Blocks   Id  System

/dev/sda1               1       59769   480092160   83  Linux

/dev/sda2           59769       60802     8291329    5  Extended

/dev/sda5           59769       60802     8291328   82  Linux swap / Solaris



Command (m for help):