Hi Rahul,

Sorry for this issue you are facing.

How Global Search Works:

Available in: All Editions except Database.com

Global search searches more record types, including articles, documents, products, solutions, and Chatterfeeds, files, groups, and people. Global search also searches more field types, including custom fields and long text fields such as descriptions, notes, and task and event comments. Global search keeps track of which objects you use and how often you use them, and arranges the search results accordingly. Search results for the objects you use most frequently appear at the top of the list. If global search doesn’t have enough information about which objects you use, you see results for your previous saved scope or for all objects until it has more information.
How your global search works depends on your search options, search terms, and the use of wildcards and operators.

Search Options:
Search options let you restrict your search to the records you own, if available; to exact phrase searches; and to divisions, if your organization uses them.

The search options you select are saved until you change them.

Note:
Chatter feed searches aren’t affected by your search scope; Chatter feed search results include matches across all objects.
Search options aren't available to Chatter Free users.

Search Terms:
Search terms are treated as separate words and may be found in different searchable fields within a record. For example, searching for bob jones returns items with Bob Jones, as well as a contact named Bob Smith whose email address is bsmith@jones.com.
Search terms are separated by letter, number, and punctuation boundaries. For example, searching for acct!4 returns items with Acct, !, and 4, even if those terms are in separate places in the item.

Tip:
If you'd like to conduct a phrase search to match multiple terms in the exact sequence that they appear, select Exact phrase in the Options or use quotations marks around your search terms.

Wildcards and Operators:
You can use the * (asterisk) and ? (question mark) wildcards to refine results. Use * to match one or more characters, or ? to match a single character. For example, searching for bob jo* finds items with Bob Jones and Bob Johnson, and searching for jo?n finds items with john and joan. You can also use the AND, OR, AND NOT, ( ) (parentheses), and " " (quotation marks) operators to refine results.

For more information with similar discussion please refer to below links.

• https://success.salesforce.com/ideaView?id=08730000000jIdDAAU
• https://help.salesforce.com/articleView?id=000194065&type=1

Hope this helps.

Kindly mark this as solved if the reply was helpful.

Thanks,
Nagendra