I have a custom object. From Campaign I have a lookup to the Custom object. What I want is to create a roll-up summary like feature on the Custom object but I can't use apex code. Can anyone suggest me if there is a way to achieve this using Process Builder or Workflow?
I'm not aware of simple way to do it with Process Builder or Workflows, but if don't have the option of converting it to a Master-Detail relationship, and if you can't use Apex code, then you have a couple options:
If you want to create a Flow, check out the Salesforce Weekly how-to article mentioned on this Community answer.
If you are looking for a ready-made solution, check out the suggestions on this question. I believe you can find the one @Eric is referring to is by following the GitHub link in @crmprogdev's answer (there is a direct link to the AppExchange listing in the Features Summary section of the README). It seems the listing is private so you won't be able to search/browse for it.
I'd post the direct links myself, but I don't currently have enough rep to post more than two. So I hope this helps.
You can install an application from appexchange to enable roll up feature for a look up relationship, it has a limitation though however its free on a positive side.
Secondly what worked for me was a simple report dashboard on the parent object.
Instead of making a field i showed the information through a summary report which i inserted to a chart and finally to the page layout of parent in lookup relationship.