user26722: I downloaded Bitcoin Core (64 bit) and I am still waiting for it to synchronize. But just now, when I open the software, it loads from 1% to 100% and then a message appears with the inscription 

Cannot obtain a lock on data directory C:/Users/ADMIN/AppData/Roaming/Bitcoin. Bitcoin Core is probably already running.

Then the program disappears to the task bar and I can't open it from the task bar. Please, any help?

user41755: You must have forcibly shut down the bitcoin client previously, or maybe the daemon is still running and you are attempting to run another client listening at the same port.
To kill all bitcoin client instances, you need to know the processes listening to the port.
Open a command prompt with administrator privileges and type in the following command to get the details of services running
netstat -o

This will list all the services listening to the various ports. Find the one with the bitcoin service (you can also see the port, 8332 for the actual bitcoin client and 18332 for the testnet client). Once you have acquired the process IDs, you need to forcibly kill the processes by typing in the command
Taskkill /PID <process_id_here> /F

Try restarting the client.
Useful links I used as source: 
netstat
Killing Processes

user2306: Try opening your data directory and deleting the file .lock
Make sure Bitcoin is closed while you're doing this.

26722: <p>I downloaded Bitcoin Core (64 bit) and I am still waiting for it to synchronize. But just now, when I open the software, it loads from 1% to 100% and then a message appears with the inscription </p>

<blockquote>
  <p><code>Cannot obtain a lock on data directory C:/Users/ADMIN/AppData/Roaming/Bitcoin. Bitcoin Core is probably already running.</code></p>
</blockquote>

<p>Then the program disappears to the task bar and I can't open it from the task bar. Please, any help?</p>
 41755: <p>You must have forcibly shut down the bitcoin client previously, or maybe the daemon is still running and you are attempting to run another client listening at the same port.</p>

<p>To kill all bitcoin client instances, you need to know the processes listening to the port.</p>

<p>Open a command prompt with administrator privileges and type in the following command to get the details of services running</p>

<pre><code>netstat -o
</code></pre>

<p>This will list all the services listening to the various ports. Find the one with the bitcoin service (you can also see the port, <code>8332</code> for the actual bitcoin client and <code>18332</code> for the testnet client). Once you have acquired the process IDs, you need to forcibly kill the processes by typing in the command</p>

<pre><code>Taskkill /PID &lt;process_id_here&gt; /F
</code></pre>

<p>Try restarting the client.</p>

<p>Useful links I used as source: </p>

<p><a href="https://stackoverflow.com/questions/258136/investigating-which-windows-service-is-listening-to-which-ip-and-port">netstat</a></p>

<p><a href="http://tweaks.com/windows/39559/kill-processes-from-command-prompt/" rel="nofollow noreferrer">Killing Processes</a></p>
 2306: <p>Try <a href="https://bitcoin.stackexchange.com/questions/4006/where-is-the-data-folder-for-bitcoin-qt">opening your data directory</a> and deleting the file <code>.lock</code></p>

<p>Make sure Bitcoin is closed while you're doing this.</p>
