<p>I am using Oracle 10g R2. 
Recently, after rebooting the server, I started having a problem where I couldn't connect to the instance. I am only connecting locally on the server itself. </p>

<p>Oddly enough, the issue corrects itself if I start the Database Administration Assistant, and select my instance to supposedly change its settings. </p>

<p>Does anybody have a clue on the roots of this problem?</p>

<p>@akaDruid: I am testing my connection simply by trying to start SQLPlus on the server. </p>

<p>@Matthew: It's Windows</p>

EDIT:  I don't think I read your question properly: The listener should not affect connections on the local machine, so you can probably ignore the rest of the answer, unless it gives you a hint!  How were you testing your connection? Was ORA-12514 the only error?
(I'm assuming you're on Windows here)
I guess the listener is not starting automatically when you reboot the server, and it's getting starting in oracle administration assistant - I don't use that tool unfortunately so couldn't say.
Next time you reboot, before starting oracle administration assistant, open a command prompt and type lsnrctl status.  If the listener has not yet started you will get something like this:
```
```
C:\Documents and Settings\user>lsnrctl status

LSNRCTL for 32-bit Windows: Version 10.2.0.1.0 - Production on 27-OCT-2008 14:00:21

Copyright (c) 1991, 2005, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC01)))
TNS-12541: TNS:no listener
 TNS-12560: TNS:protocol adapter error
  TNS-00511: No listener
   32-bit Windows Error: 2: No such file or directory
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=server.domain.co.uk)
(PORT=1521)))
TNS-12541: TNS:no listener
 TNS-12560: TNS:protocol adapter error
  TNS-00511: No listener
   32-bit Windows Error: 61: Unknown error

C:\Documents and Settings\user>lsnrctl status
```
```
if it is running, you will get something like this:
```
```
LSNRCTL for 32-bit Windows: Version 10.2.0.1.0 - Production on 27-OCT-2008 14:03
:33

Copyright (c) 1991, 2005, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC01)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for 32-bit Windows: Version 10.2.0.1.0 - Production
Start Date                27-OCT-2008 14:03:27
Uptime                    0 days 0 hr. 0 min. 5 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Parameter File   C:\oracle\product\10.2.0\db_1\network\admin\listener.ora
Listener Log File         C:\oracle\product\10.2.0\db_1\network\log\listener.log

Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(PIPENAME=\\.\pipe\EXTPROC01ipc)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=server.domain.co.uk)(PORT=1521))
)
Services Summary...
Service "ORCL" has 1 instance(s).
  Instance "ORCL", status UNKNOWN, has 1 handler(s) for this service...
Service "ORCL1" has 1 instance(s).
  Instance "ORCL1", status UNKNOWN, has 1 handler(s) for this service...
Service "PLSExtProc" has 1 instance(s).
  Instance "PLSExtProc", status UNKNOWN, has 1 handler(s) for this service...
The command completed successfully

C:\Documents and Settings\user>
```
```
If the listener is not starting, check that service is set to automatic.  If it is, check the listener.ora makes sense, see what output you get from lsnrctl start, etc.
Hope that helps, or at least sends you down the right path