I won't bore you with the details, so let's just say that for some reason I don't have the owncloud admin password anymore.
I have spend lots of time on owncloud forum and found few more solution. In my point of view the below one is very simple and effective.

• Get the passwordsalt

[codesyntax lang="bash"]

server owncloud # grep passwordsalt config/config.php 
  'passwordsalt' => 'ZnuaO2o4s3Qydg5xvR4gk7yZQn7v.L',

[/codesyntax]

• Prepare the "hack"

[codesyntax lang="bash"]

server owncloud # cd /tmp/
server tmp # wget -c "http://cvsweb.openwall.com/cgi/cvsweb.cgi/~checkout~/projects/phpass/PasswordHash.php"
server tmp # wget -c "http://cvsweb.openwall.com/cgi/cvsweb.cgi/~checkout~/projects/phpass/test.php"
server tmp # sed -e "s/$t_hasher = new PasswordHash(8, FALSE);/$t_hasher = new PasswordHash(8, CRYPT_BLOWFISH!=1);/g" -i test.php
server tmp # sed -e "s/$correct = 'test12345'/$correct = 'admin123'.'ZnuaO2o4s3Qydg5xvR4gk7yZQn7v.L';/g" -i test.php

[/codesyntax]

• Run the test.php file

[codesyntax lang="bash"]

server tmp # php -f test.php 
Hash: $2a$08$sIE2IL4xZwADAqpdGeLY7.QOYBC01x7U3IKE/YS6XZ1n.TVd1jnTS
Check correct: '1' (should be '1')
Check wrong: '' (should be '0' or '')
Hash: $P$BdVJYUfc8uplEowbiO3WWPRKXLLLY..
Check correct: '1' (should be '1')
Check wrong: '' (should be '0' or '')
Hash: $P$9IQRaTwmfeRo7ud9Fh4E2PdI0S3r.L0
Check correct: '' (should be '1')
Check wrong: '' (should be '0' or '')
Some tests have FAILED

[/codesyntax]

• Update the new password to admin user via MySQL query

[codesyntax lang="bash"]

root@localhost [(none)]> use owncloud;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
root@localhost [owncloud]> update oc_users set password="$2a$08$sIE2IL4xZwADAqpdGeLY7.QOYBC01x7U3IKE/YS6XZ1n.TVd1jnTS" where uid="admin";
Query OK, 0 rows affected (0.13 sec)
Rows matched: 1  Changed: 0  Warnings: 0
root@localhost [owncloud]>

[/codesyntax]

• Now, you have successfully reset your owncloud password, just navigate on your owncloud installation url on browser and login into your admin account using your new password (in my case new password is admin123)