Every time I install a new extension from Magento commerce and I return to admin from Magento connect it shows Fetal error class not found in mage.php
please help me with this.
Fatal error: Class 'Qualityunit_Liveagent_Helper_Data' not found in C:\wamp\www\magento\app\Mage.php on line 548
This issue arise only in two cases.
1) You are creating any custom module in magento.
2) Your compiler having same issue.
In your case you are facing issue with default magento extension installation. So it should be second option. Here is solution for second case.
From System, Tools disable or re-compile Compilation. If you can't access to admin interface you can disable it by SSH:
php -f shell/compiler.php -- disable
php -f shell/compiler.php -- clear
php -f shell/compiler.php -- state
The final output should appear like:
Compiler Status: Disabled
Compilation State: Not Compiled
Collected Files Count: 0
Compiled Scopes Count: 0
For First case, you will surely get help from here.
AFAIK ,
there are 2 other possibility for this error except compiler issue discuss by @Pankaj.
this class does not exist.
for 1st case check file should be at there.
app/code/local(or)community/Qualityunit/Liveagent/Helper/Data.php
this Data.php file should be this location
2Nd possibility
May be your module is disable from System->Configuration->Advanced->Advanced
check if this module is disable
OR check
app/etc/modules/Qualityunit_Liveagent.xml
In this file check if <active>false</active> is there or not if this node is false then set true and check once again
