Just restart your machine and follow the steps:

1. At the boot(grub) menu select the linux kernel

2. then press e

3. Go to the end of the line

4. add 1 or S or Single at the end of the line

5. press enter

6. press b on the line to boot from that line

7. Now the root prompt will come

8. run the command 'passwd' and it will ask you to change your password for root