An Error has occurred while encrypting connection string

I am trying to decrypt connection string in my app.config using customized Config Protection Provider named "MyConfigProtectionProvider". I don't have code for this Customized Config Provider as it is developed a long back, I have only DLL file. We changed our DB Username and Password. I want to decrypt my connection string in Web.Config using this customized Config Protection Provider and change password to new password and encrypt it again

I have only app.config. So I renamed it to "web.config". Then I added CODE I as shown below in my web.config. I store key in my c drive and I need to give Key file path. "MyConfigProtectionProvider" will encrypt it using this key.

We already have encrypted connection string as shown in CODE II. So I tried decrypting it using following command in cmd as Admin

aspnet_regiis.exe -pdf "connectionStrings" "C:\Users\Tejas\Documents\Email_Notification\LatestBatchJobStatus\BatchJobStatus\Batch_Job_Status"

  -prov "MyConfigProtectionProvider"

CODE I:

<configProtectedData defaultProvider="MyProtectionConfigurationProvider">
<providers><add name="MyProtectionConfigurationProvider" type="MyProtectionConfigurationProvider,&#xD;&#xA;MyProtectionConfigurationProvider, Version=1.0.0.0,Culture=neutral,&#xD;&#xA;PublicKeyToken=#############, processorArchitecture=MSIL"keyFilePath="C:\Users\Tejas\Documents\Email_Notification\LatestBatchJobStatus\BatchJobStatus\Batch_Job_Status\ConnectionKey\ConnectionKey.txt" /></providers></configProtectedData>

CODE II:

<connectionStrings configProtectionProvider="MyProtectionConfigurationProvider">  <EncryptedData>7Zqa4I623WR..................</EncryptedData></connectionStrings>

I am getting following error "An error has occurred: 0X80070002 The System cannot find the file specified".

What is the mistake I made?