In Notepad++:

• Find what: [\n\r]+
• Replace with: ,
• Check the “Wrap around” checkbox
• In Search mode, check “Regular expression”, and check “, matches newline”
• Use “Replace All” button

Explanation:

• [\n\r]+ finds 1+ occurrences of newlines and linefeeds
• , replaces what you have found with comma & space