Suddenly can't login on my site as admin?

There are some tips that you can check before :

1. What happen when you try to login with admin ? Any system warning / message ?
2. Do you have access to your database (via phpmyadmin p.e) ?
3. Is there admin user in users table? Which status has (0 or 1)?
   Inspect flood table cause maybe your admin user can be blocked (as you can see in this article http://geroldm.com/2011/10/drupal-sorry-there-have-been-more-than-5-fail...)
4. Maybe you find an answer in your server logs.

I think that I'll need more information for help you and find what is wrong.
Regards

please try to upload new php.ini

don't upload few like this:-

memory_limit = 70M
post_max_size = 804M
file_uploads = On
upload_max_filesize = 200M
max_execution_time = 140

upload all php.ini files

it's work for me after upload new php.ini

I'm running a D7 site, but this is likely still something that could effect D6.

Most of my users are anonymous and do not use any accounts. I have 1 page that is restricted to internal employees and used to stream video of odd events or our quarterly performance meetings. My instruction was to create a single, shared account for all these users.

Just today we had a big announcement and combined with recent headcount growth, had more concurrent users than at any other time. I started receiving emails that some users were being denied access, but knews others using the same account were being permitted. I logged out of my admin account, confirmed I could not log in using the shared account, then was unable to log in with my admin credentials. Panic!

After restarting Apache (a prayer to the system gods) I received a notice in my apache2 error.log that I had hit my MaxClients setting. Since hosting these events is a secondary function of my server I notified my internal uses to sit tight. Once the event ended and most of my connected users logged out I found I was able to get back into my admin account.

So this weekend I'll be looking at my Apache settings.

Hope this helps.

On the web server:

# drush sqlq "DESCRIBE sessions;"                                                   
Query failed.  [error]

# drush sqlq "SELECT count(*) FROM sessions;"
Query failed.  [error]

Then on the database server, log in as root; . . .

mysql> REPAIR TABLE sessions;
+-------------------+--------+----------+-------------------------------------------------------+
| Table             | Op     | Msg_type | Msg_text                                              |
+-------------------+--------+----------+-------------------------------------------------------+
| ggp_cms6.sessions | repair | info     | Found block that points outside data file at 16322192 |
| ggp_cms6.sessions | repair | warning  | Number of rows changed from 111884 to 111868          |
| ggp_cms6.sessions | repair | status   | OK                                                    |
+-------------------+--------+----------+-------------------------------------------------------+
3 rows in set (4 min 36.00 sec)

mysql> SELECT count(*) FROM sessions;
+----------+
| count(*) |
+----------+
|   111875 |
+----------+
1 row in set (0.34 sec)

mysql> DELETE FROM sessions;
Query OK, 111875 rows affected (1.27 sec)

-- Hugh Esco

Make sure whether the apache rewrite module is enable and working properly.

Yes, despite the .htaccess file being present with the require rewriting rules, apache mod_rewrite still has to be enabled on the server (if you are using apache).

Explicitly, you have to issue the command:
sudo a2enmod rewrite

Then restart apache

Check settings.php, and remove script if you wrote for configuring of any caching modules and any other module. In my case there was wincache module setting script there.. Initially I ignore this, but that was the culprit. I wasted three day in this issue..